It is known to produce a polyolefin using a Ziegler Natta catalyst comprising a solid catalyst component composed of a titanium component and a magnesium halide carrier, and an organic aluminum compound component as described in JP-A-39991/78, JP-A-206415/84, etc.
This conventional catalyst has a relative high activity. However, there is a problem that when the polyolefin produced by the above-described conventional method is molded using a molding machine, magnesium halide which remains in the polyolefin produced promotes corrosion of the molding machine and leads to production of molded articles having a poor appearance.
On the other hand, a method for producing a polyolefin has been proposed which uses a solid catalyst component prepared by supporting a magnesium dialkoxide on a carrier of an inorganic oxide such as SiO.sub.2 not containing a halogen and reacting the thus-prepared solid catalyst component with an electron donating compound such as a carboxylic acid monoester and a titanium halide compound, as described in JP-A-162607/83.
However, this method is disadvantageous in that the catalyst used has a low catalytic activity and the deposition of the magnesium alkoxide on the carrier is insufficient, resulting in that the polyolefin obtained is powder of poor particle size distribution and has an insufficient stereoregularity, and that depending on the kind of the .alpha.-olefin used the molecular weight distribution of the resulting polyolefin is broad and films molded therefrom have a poor transparency.
Further, a method for producing a polyolefin using a solid catalyst composed of a titanium halide and a carrier which is prepared by reacting an organic silicon compound with silica, for example, and further reacting the product with a magnesium compound such as a magnesium alkyl halide compound (RMgX) and with an alcohol, sequentially, as described in Japanese Patent Publication No. 6962/85.
This conventional method, however, is disadvantageous in that polymers having a high stereoregularity are not obtained.
On the other hand, it is known to produce a polypropylene, one of polyolefins, using a multi-stage polymerization method in order to improve the impact strength of the polypropylene.
For example, there is known a two-stage slurry polymerization method as described in , e.g., Japanese Patent Publication No. 11448/83. The polypropylene produced by this method has an impact strength which is improved to some extent but is insufficiently satisfactory.
A three-stage slurry polymerization method is also proposed as described in Japanese Patent Publication No. 145114/82, which method fails to produce polypropylenes that have a satisfactorily improved impact strength.
In addition, a two-stage gas phase polymerization method using a titanium trichloride catalyst is proposed as described in, e.g., JP-A-187 413. Although this method is somewhat improved since it does not require a step of recovering the solvent used and thus a step of drying the resulting polymer is widely simplified, it still needs to remove catalyst residue because the catalyst used has a low catalytic activity. In order to cope with this problem, development of a gas phase polymerization method using a catalyst having a high catalytic activity is under way but there are many problems to be solved before such method can be utilized in practice and a complete solution therefore has not been proposed yet as far as is known.
Among polyolefins, there is included propylene ethylene random copolymers, which are used widely in various fields of film technology. The propylene-ethylene random copolymers are produced by copolymerizing propylene and ethylene monomers using a stereoregular polymerization catalyst comprising a solid catalyst component composed of a titanium compound supported on a magnesium halide carrier, and an organic aluminum compound.
However, such propylene-ethylene random copolymers are disadvantageous in that they have poor heat sealing properties at low temperatures.
In order to improve the heat sealing properties of propylene-ethylene random copolymers, it has conventionally been proposed to increase the content of ethylene as a monomer unit.
When a slurry polymerization method is employed in order to increase the content of ethylene, not only the amount of soluble polymers which are by-produced increases and as a result the yield of the propylene-ethylene random copolymer desired decreases but also the properties of the slurry during the polymerization reaction is deteriorated and it is difficult to carry out the polymerization reaction continuously.
On the other hand, an attempt to increase the content of ethylene by the use of a gas phase polymerization method gives rise to a new problem in that the resulting propylene-ethylene random copolymer has an increased adherent property which tends to lead to the formation of agglomerates and as a result to the occurrence of accidents such as clogging of pipelines for transportation and the like.
Further, the various polyolefins produced by the above-described polymerization methods, which are usually transported in a pipeline to a predetermined place in the form of dry powders, tend to cause accidents due to clogging of pipelines for transportation since the polyolefins produced by the conventional methods have unsatisfactory particle size distribution.